The present invention relates to an improvement on the apparatus and methods described and claimed in U.S. Pat. No. 4,489,432 and in U.S. Pat. No. 4,638,505, which relate to arrangements for reproducing sound and obtaining an enhanced acoustic image. In accordance with one aspect of the present invention, it relates to reproducing sound and obtaining an enhanced acoustic image in the context of an automobile sterephonic sound reproduction system. One aspect of the method and apparatus described in U.S. Pat. No. 4,489,432, the disclosure of which is hereby incorporated by reference, is the requirement that a left minus right signal (L-R) be developed and fed to the left sub-speaker and that a right minus left signal (R-L) be developed and fed to the right sub-speaker. The application of these signals to the sub-speakers in combination with the other aspects of the invention produce an expanded acoustic image.
U.S. Pat. No. 4,638,505, the disclosure of which is also hereby incorporated by reference, represents an improvement to the basic system and method of U.S. Pat. No. 4,489,432, with specific reference to optimizing the low frequency response of such loudspeaker systems which have main and sub-speakers. Specifically, the required signals for the sub-speakers are derived in such a way that they will work in concert with the associated right or left main speaker at low frequencies regardless of the relationship of right to left channel stereo signals coming from the amplifier. Inter-speaker cabling is used for development of the R-L and L-R signals for application to the sub-speakers. The inter-speaker cabling includes impedance means such that above approximately 200 Hz the right and left sub speakers primarily receive, respectively, the R-L and L-R signals, and below approximately 200 Hz the right and left sub speakers primarily receive respectively, the R and L signals.
Regardless of placement, the addition to an automobile sound system of a pair of speakers producing only difference signals would add considerable ambience and reverberance to the automobile sound system. U.S. Pat. No. 3,697,692 to Hafler describes such a system for home use. However, if Hafler's system were adopted for use in an automobile the rear speakers would produce very little bass information since it is not normally present in the difference signal. Since most automobile sound systems rely on the rear speakers (which typically use the air volume of the automobile trunk), for most of the low frequency output, this would be a serious limitation.
U.S. Pat. No. 4,594,729 to Weingartner relates to an automobile sterephonic sound system in which a pair of rear speakers are provided and R-L and L-R signals applied thereto. In that arrangement differential amplifiers are apparently used to derive the difference signals. Various attenuators also appear to be employed for increasing the amount of the L portion in the L-R difference signal applied to the left rear speaker and the R portion in the R-L difference signal applied to the right rear speaker. This may be an attempt to deal with the lack of bass output from speakers fed only by difference signals, but it is at best complicated and expensive.